Cryogenic air separation methods have been widely used as methods for separating and recovering oxygen and nitrogen, which are contained in air, from air as a feed gas. In these cryogenic air separation methods, feed air is liquefied and oxygen and nitrogen are distilled by utilizing the difference in boiling points of nitrogen and oxygen. Since this cryogenic air separation method has high system costs, it is suitably utilized in a system consuming a large amount of products.
A pressure swing adsorption method (abbreviated as “PSA method” below) utilizing an adsorbent for separation has been recently used for separation and recovery of air components. When oxygen is separated and recovered as a product from air by this PSA method, zeolite is used as an adsorbent. When pressurized air is introduced into an adsorption column filled with zeolite, nitrogen, which is easily adsorbed by zeolite, is adsorbed and held by zeolite, and oxygen, which is not readily adsorbed by zeolite, passes through the adsorption column and flows out (an adsorption step). Then, when the pressure in the adsorption column filled with the zeolite adsorbent decreases until it is less than the pressure in the adsorption column during the adsorption step, nitrogen, which is adsorbed and held by the zeolite adsorbent during the adsorption step, is desorbed and flows out from the adsorption column and recovered (a regeneration step).
In the PSA method, the adsorption step, which is performed under relatively high pressures, and the regeneration step, which is performed under relatively low pressures, are alternately repeated in a short time. Therefore, it is easy to increase the recovery amount of a desired product per unit mass of the adsorbent, and the system for the PSA method can be reduced in size.
For example, in a PSA method, when the adsorption step and the regeneration step are alternately performed using two adsorption columns, the desired product can be successively recovered from the two adsorption columns. In almost all PSA methods, a component, which is not readily adsorbed by the adsorbent and passes through and flows out of the adsorption column, is recovered with high purity. When a component, which is easily adsorbed by the adsorbent, is recovered as a desired product, the adsorbent adsorbs the component, which is easily adsorbed, but the adsorbent also simultaneously adsorbs a small amount of other components, which are not readily adsorbed. In addition, the other components, which are not readily adsorbed, remain in void spaces between the adsorbents. During the regeneration step, in which the pressure in the adsorption column is decreased, the other components in the adsorption column, which are not readily adsorbed, are caused to flow out together with the desorbed gas. Therefore, the component of products, which are easily adsorbed, contain a considerable amount of the other components, which are not readily adsorbed. Moreover, a mixing ratio between the components, one of which is easily adsorbed and the other of which is not readily adsorbed and which flow out from the adsorption column during the regeneration step, is determined depending on the property of use adsorbents.
Therefore, When the component which is easily adsorbed is recovered as a product with high purity, during the regeneration step before the adsorption step, it is necessary to purge the adsorption column which is condition in the end of the adsorption step using the component which is easily adsorbed and is recovered as a product (which is recovered by desorbing in the previous step). This leads to complications in the separation and recovery steps. In addition, this also requires the provision of further equipment for purging, such as compressors.
In order to recover a desired gas product having high purity, a conventional device and method for separating gas based on the PSA method, mainly targets only one component contained in a gas mixture containing a plurality of component gases.
For example, Unexamined Japanese Patent Application, First Publication No. Sho 55-147119 discloses a PSA method for separating and recovering each of the plurality of components with high purities from a gas mixture. This separation method utilizes an adsorption system comprising a preliminary column filled with an adsorbent which selectively adsorbs H2O and CO2 contained in air, and a main column filled with another adsorbent which selectively adsorbs nitrogen rather than oxygen. This separation method provides oxygen enriched air and nitrogen with high purity.
A conventional method for separating air based on a PSA method disclosed in Unexamined Japanese Patent Application, First Publication No. Sho 55-147119 is explained below with reference to FIG. 3.
The PSA air separation system 50 comprises preliminary columns Y1 and Y2 filled with an adsorbent, which selectively adsorbs H2O and CO2, and main columns Z1 and Z2 filled with another adsorbent, such as zeolite, which selectively adsorbs nitrogen. These columns form two pairs of treatment systems comprising the preliminary column Y1 and the main column Z1, and the preliminary column Y2 and the main column Z2. These treatment systems are connected so as to be provided with feed air alternately. While one treatment system is provided with feed air and oxygen enriched air is yielded by adsorbing the nitrogen (an adsorption step), the other treatment system desorbs the nitrogen which is adsorbed by the adsorbent in the previous adsorption step, and provides nitrogen having high purity (a regeneration step).
For example, when the combination comprising the preliminary column Y1 and the main column Z1 is in the adsorption step and the other combination comprising the preliminary column Y2 and the main column Z2 is in the regeneration step, the system comprising these combinations is operated as follows.
When the treatment system comprising the preliminary column Y1 and the main column Z1 is in the adsorption step, a feed air MA is introduced into the pipeline 51, passed through the valve V51 and the blower 52, thereby the feed air is pressurized to a predetermined pressure, and the pressurized feed air is passed through the pipeline 53, the valve V52 and pipeline 54, and is supplied into the preliminary column Y1. In the preliminary column Y1, H2O and CO2 are adsorbed and removed from the feed air. Then, the air M, from which H2O and CO2 are adsorbed and removed, is passed through the valve V53 and is then introduced into the main column Z1 filled with the zeolite adsorbent. While passing through the main column Z1, the nitrogen component contained in the air M is adsorbed by the zeolite adsorbent. The oxygen, which is not easily adsorbed by the zeolite adsorbent, is concentrated and becomes an oxygen enriched air. The oxygen enriched air is caused to flow into the pipeline 55, pass through the valve V54 and the pipeline 56, and is recovered in the oxygen enriched air tank 57. After that, the oxygen enriched air is supplied into a point of use, via the pipeline 58.
When the treatment system comprising the preliminary column Y2 and the main column Z2 is in the regeneration step, the valve V65 is opened, and thereby nitrogen gas containing the oxygen accumulated in the main column Z2 is caused to flow out into the pipeline 65. Then, the nitrogen gas containing the oxygen is passed through the valve V65 and the pipelines 69 and 60, and is recovered in the exhaust gas tank 61. The exhaust gas is used for regeneration cleaning of the preliminary columns Y1 and Y2. After that, the nitrogen adsorbed by the adsorbent in the main column Z2 is desorbed by closing the valve V65 and opening the valves V63 and V66 and operating the vacuum pump 62. The desorbed nitrogen is passed through the valve V63 and the preliminary column Y2, the pipeline 64, the valve V66, and the pipelines 63 and 67 via the vacuum pump 62, and is then recovered in the nitrogen storage tank 68 as a nitrogen gas product having high purity. After that, the nitrogen gas product is supplied into a point of use, via the pipeline 70.
As explained above, the treatment systems comprising the preliminary column Y1 and the main column Z1, and the preliminary column Y2 and the main column Z2 are alternately in the adsorption step and the regeneration step by operating the valves. In this method, in order to yield the nitrogen gas product having high purity, after the adsorption step and before the regeneration step, a cleaning step is performed. In the cleaning step, the recovered nitrogen is caused to flow out of the nitrogen storage tank 68 and is introduced into the pipeline 71, the valve V67, the pipeline 72, the circulation blower 73, the pipeline 74, and the valves V75 and V76, and is supplied into the main column Z1 or Z2. The introduced nitrogen gas cleans the main column Z1 or Z2, that is, the nitrogen concentration in the main column Z1 or Z2 is increased by introduction of nitrogen gas. Thereby, the purity of the nitrogen gas product is increased.
However, during the regeneration step, nitrogen for desorbing is introduced into the main column Z1 and Z2 in a flow direction which is opposite from the flow direction of the feed air during the adsorption step. In other words, nitrogen which is desorbed from the main column Z1 and Z2 has high purity, but in the regeneration step, since this nitrogen passes through the preliminary column Y1 or Y2 absorbing H2O and CO2 and this nitrogen flows out together with H2O and CO2. Therefore, the nitrogen contains a high percentage of H2O and CO2 as impurities. Therefore, it is necessary for these impurities to be removed, this leads to the need for the provision of further equipment, for example, additional blowers, air flow devices, and refining devices, such as dryers. Due to this, the separation system is complicated and the system cost also increases.
As explained above, in the air separation method disclosed in Unexamined Japanese Patent Application, First Publication No. Sho 55-147119, the exhaust gas containing oxygen at a relatively high concentration, which is exhausted by cleaning the main columns using the nitrogen product, is recovered by passing the pipelines used for introducing the feed air. In contrast, in the air separation method disclosed in Unexamined Japanese Patent Application, First Publication No. Hei 1-297119, the exhaust gas containing oxygen with a relative high concentration is recovered as a part of the oxygen enriched air product. These methods also have the problem that the nitrogen product is contaminated with H2O and CO2 and the system is complicated.
Unexamined Japanese Patent Application, First Publication No. Sho 52-52181 discloses a recovery method for oxygen gas having a high purity of 99% or greater in which an adsorption column filled with a molecular sieve carbon (abbreviated as “MSC” below) and an adsorption column filled with a zeolite are connected, and feed gas is caused to flow through these columns successively in contacting. In general, when oxygen is recovered from air by zeolite, zeolite does not adsorb with argon very much and therefore, the oxygen product is contaminated with argon. Therefore, the oxygen concentration after adsorption and separation is less than about 95%. In this method, in order to yield an oxygen product having a concentration of 95% or more, two kinds of the adsorbents are used, depending on the composition of the feed gas.
In this adsorption and separation method disclosed in Unexamined Japanese Patent Application, First Publication No. Sho 52-52181, the gas mixture, which is caused to flow out from the first adsorption column filled with the first adsorbent and which contains the main gas component with a high percentage content is used as a feed gas for the second adsorption column filled with the second adsorbent, or is recovered as a gas product. Therefore, a pump unit for sending the desorbed gas from one adsorption column to the other adsorption column or for circulating the desorbed gas through the adsorption column is required. The system is complicated, and the system cost is increased.
As explained above, almost all of the conventional gas separation methods and systems which are based on PSA methods separate and recover only one component in the raw gas mixture with high purity. A separation method and system which can yield a plurality of components with high purity as products has been desired.
In consideration of the above-described problems, it is an object of the present invention to provide a gas separation method and a system based on a PSA method, in which a plurality of components contained in a gas mixture can be separated and recovered at high purities at the same time, the system being simple, the system cost being low, and the operation being easy.